Process for fabricating a fission product retentive nuclear fuel body



Dec. 20, 1966 Original Filed March B. E. INGLEBY PROCESS FOR FABRICATING A FISSION PRODUCT RETENTIVE NUCLEAR FUEL BODY 2 Sheets-Shea*l l DCC- 20, 1956 B. E. INGLEBY PROCESS FOR FABRICATING A FISSION PRODUCT RETENTIVE NUCLEAR FUEL BODY Original Filed March 29, 1965 2 Sheets-Sheet 2 United States Patent O 3,293,332 PROCESS FOR FABRICATING A FISSION PROD- UCT RETENTIVE NUCLEAR FUEL BODY Bryan Edward Iugleby, Salwick, Preston, England, assignor to United Kingdom Atomic Energy Authority, London, England Original application Mar. 29, 1963, Ser. No. 269,084. Divided and this application Feb. 3, 1965, Ser. No. 437,342 Claims priority, application Great Britain, Apr. 13, 1962, 14,475/62; Mar. 11, 1963, 9,538/63 4 Claims. (Cl. 264-5) This is -a division of application Serial No. 269,084, filed March 29, 1963.

This invention relates to a proces for production of that kind of nuclear reactor fuel element which is intended to operate in contact with `fiuid coolant and to retain iission products which are produced as a result of irradiation of the fuel in a nuclear reactor, whereby contamination of the coolant by the fission products is avoided.

According to the invention, there is provided a process `for producing `fuel bo-dies incorporating sintered uranium carbide spheroids, each spheroid being individually coated with a substantially uniform coating of .silicon carbide, and the coated spheroids being ydispersed in a matrix of silicon carbide, such process including the steps of .granulating mixed uranium `dioxide and carbon, spheroidising the granules so lformed, effecting reaction-sintering of the spheroids thus formed so as to convert them to spher-oids of uranium carbide, coating the uranium carbide spheroids with `silicon carbide, mixing the coated spheroids with silicon carbide and carbon, extruding the mixture to form compa-cts, and effecting self :bonding of the compacts by effecting reaction between said last-added carbon and silicon to produce dense fuel bodies.

In order that the invention may be fully understood and more readily carried into practice, a process for producing shaped `fuel bodies will not be described with reference to the accompanying drawings, wherein FIGURE 1 is a iiow sheet illustrating the various steps of the process which will hereafter be described in some detail, and

FIGURE 2 is a front view in medial section of apparatus for performing one step of t-he process.

The process for producing the fuel bodies comprises the following stages: rstly producing .spheroids of mixed uranium dioxide and carbon, together with a `suitable quantity of `a binder, secondly reaction-sintering such spheroids to form dense sintered spheroids of uranium carbide, thirdly coating such .sintered spheroids with silicon carbide, ,[ourthly dispersing such coated -spheroids in a silicon carbide matrix, and lastly :forming the matrix containing the dispersed coated spheroids to a desired shape of high density.

The `first stage is performed by milling, with steel balls, a mixture of ceramic grade uranium dioxide (surface area 3 m.2/.gra.m), carbon :black (surface area 17 m.2/gram) and aluminium stearate binder in a rub-ber or rubberlined pot for 16 hours. The r-atio of uranium dioxide to carbon is about 123.02, that is slightly in excess of the st-oichiometric ratio to produce uranium monocarbide by the 'formula A typical example is 500 grams U02 and 67 grams carbon black, together with 0.25% by weight aluminium stearate.

3,293,332 Patented Dec. 20, 1966 rice For the preparation of uranium dicarbide, the following formula applies:

The ratio of U02 to carbon is now about 124.05 and a typical example is 500 grams U02 land 90 `grams carbon black. Although the use of uranium monocarbide appears to have some advantage over the use of uranium dicarbide as nuclear fuel, the latter is by no means inapplicable, and references to uranium monocarbide in the ensuing description .should be understood to include uranium dicarbide as an alternative. The milling produces lgranules o-f a mixture of the constituents, which granules are then transferred to a polythene bowl in which they are gyrated, the Ibowl being secured to a conventional sieve shaker which imparts both reciprocatory motion in a vertical direction and oscillatory angular motion in a horizontal plane to the lbowl. Spheroids in the size 4range 20G-250 microns are produced by the gyration. Where spheroids of larger diameter are required, the spheroids can be caused to grow by addition of. more granules of the mixture resulting from ball milling to the spheroids, continuing -gyration for periods of about 1 hour and then sieving to remove .sm-aller spheroids formed by independent spheroidication of those of the lgranules which have not caused growth of the spheroids alre-ady in being when the addition was made. Such .smaller spheroids are returned to the ball mill afor reagranulation 'before constituting `subsequent additions. The spheroids are grown t-o a desired size range (for example about 700 microns), removed from the polyth-ene bowl, finally sieved to remove under-sized spheroids, and the retained spheroids are then placed -in molybdenum boats in a carbon resistance furnace under about l micron of vacuum. The temperature is taken up `from ambient at a rate of 10U-200 C./hr. until a temperature of about l360 C. is reached, reaction between the uranium ydioxide and the carbon occurring with evolution of carbon monoxide which is removed fro-m the reaction zone by the vacuum pump. When reaction has ceased (indicated by no more carbon monoxide being evolved), the tempertaure of the furnace is raised at a rate of about C./hr. up to a sintering `temperature of 17-00 C. at which the :furnace temperature is held :for about 4 hours. Cooling at a controlled rate follows, and after removal from the furnace, sieving only to break up agglomerates of spheroids is effected. The produced spheroids are ffound to have retained their spherical form and are of high density.

The uranium monocarbide spheroids are provid-ed with a coating of silicon carbide by employing for coating the vapour resulting from the `decomposition of methyl trichlor-osilane as suggested by Kendall and by Susman in separate articles contained in the text book Silicon Carbide edited by J. R. OC-onnor and J. Smiltens and published by Pergammon Press. I have given this suggestion practical form by subjecting the spheroids to the said vapour in a fluidised bed reactor. A suitable fluidised bed reactor is illustrated in FIGURE 2 of the `accompanying drawings. The fluidised bed reactor comprises a hollow cylindrical casing 1 having a side pocket 2 Ifor a thermocouple not shown) employed for monitoring heating element temperature, the casing 1 containing an annular thermal insulating structure 3 consisting of alumina bricks 4 contained in a mild steel structure 24 and provided with a molybdenum reflector 5, the structure 3 being in contact with a purge lgas, conveniently argon, introduced at inlet 6 and taken off at outlet 23. The structure 3 encompasses a graphite sleeve 7 which provides a resistance heating element lfed by a power source 8. The sleeve 7 encompasses a bed assembly -consisting of a lower hollow cylindrical part 9 of graphite, an upper cylindrical part 10 of graphite which is screwed to the part 9 at 11, and an inner hollow cylindrical part 12 of graphite which is supported at its lower end on an annular shoulder 13 of the part 9. The bore (typically 11/2 diameter) of the part 12 forms the bed chamber 20 and is mainly cylindrical but at its lower end it has a 60 coned portion 14 which forms the base of the uidised bed chamber 20 and effects outward diffusion of the uidising gas. An aperture 1S provides inlet .for the uidising and coating gases and is provi-ded with a tungsten carbide |ball 16 to act .as a valve to prevent loss of bed material when the bed is not fluidised. Fluidising and coating gases are supplied to the fluidised bed chamber 20 :by a pipe 17 extending from outside the casing to nearly the aperture .15 and provided with a water jacket 18. The temperature of the bed is monitored by a thermocouple 19, and off-.gases from the bed pass along the bore of the upper part .10 and into a water-cooled expansion header 21, from whence they are conducted to conventional scrubbing and filtering equipment (n-ot shown) via an outlet pipe 22.

The operation of the bed is as follows. The equipm-ent is iirst degassed at l000 C. in .a stream of argon, then cooled to .20G-300 C. and a charge of uranium carbide spheroids is loaded into the chamber 20. The bed is heated by means of the element 7 to 1200-1850o C., typically 1500" C., with a small flow (about 0.2 lit-re/ min.) of hydrogen passing through the bed. When the desired temperature is reached, the hydogen gas ow is increased to fluidise the bed of spheroids 1( abo-ut 20y litres/ min.) and at the same time the reactanft gas is introduced to the uidising gas stream. This is accomplished by providing a .by-pass (not shown) from the main hydrogen gas How, the bypass stream being bubbled through a vessel (not shown) containing .methyl trichlorosilane and then being conducted to rejoin the main gas stream. When the desired coating thickness (7S-100 microns) has Ibeen achieved, the .gas stream is stopped, power to the heater element 7 is switched olf, and the bed is allowed to cool, whereafter the coated spheroids are elutriated with water, dried, leached :for 8 hours in boiling M nitric acid, again elutriated with water, dried, and then returned to a clean environment (effected by replacement of parts 9, and 12 by new parts) for a further coating of silicon carbide. By means of the second coating, any uranium carbide contamination existing near or on the surface of the initially coated `spheres (due for example to fracture of one or more spheroids) is sealed by .a coating of uncontaminated .silicon carbide.

To allow a thinner silicon carbide coating to be employed without adversely affect-ing integrity, it has been found that an initial coating of pyrolytic carbon will give the desired result. This can be effected in the same bed, prior t-o silicon carbide coating, by fluidising on argon at about 5 litres/min. and adding methane or acetylene to the fluidising gas. Such a system of pyrolytic carbon and silicon carbide is known as a yduplex coating. It is also possible, if desired, to produce triplex coatings of alternate layers of silicon carbide, pyrolyti-c carbon and silicon carbide.

The coated spheroids so produced are formed into fuel bodies by being dispersed in a silicon carbide matrix as tfollows. I600 mesh (about 10u) alpha-silicon car-bide powder, ultra-tine colloidal :graphite (eg. DAG 621), and the coated spheroids in the proportion 2 parts by weight of the silicon carbide powder to 1 part by weight of the colloidal graphite and the calculated weight of coated spheroids to give a load-ing of 40 volume percent, are dry mixed in a Y-cone mixer. This mix is slurried by adding Cranco in methylethylketone, and Ian epoxy resin solution (e.=g. Araldite AY-lS-HZ-llS). The methylethylketone is allowe-d to evaporate from the slurry until the mix is of the right plasticity for extrusion. This can be judged by the extrusion pressure required for a sample 0.5 ton per sq. in. (nominal) is suitable. Typical quantities are 100 grams of the silicon carbide powder, 50 grams of the colloidal graphite, the requisite weight of spheroids (as aforesaid), ml. of the Cranco solution, and 75 m1. of the epoxy resin solution. Extrusion using hardened steel dies and a hydraulic press is then performed to produce green compacts, which may, for example, be of solid elongated cylindrical form. The compacts are loaded onto Ialumina trays to Ibe heated in an air oven, the temperature of which -is raised at the rate of 50 C./hr. to 250 C. at which temperature the oven is held for 5 hours. By this means, the epoxy resin is cured and the Cranco lubricant is evaporated olf. The compacts Iare then heated to 750 C. 'under vacuum at a rate of 50 C./hr., being held at 750 C. for 10 hours to carbonise the epoxy resin. Siliconising of the compacts is -next performed under vacuum in a high .frequency induction heater, the compacts being stood in a graphite Crucible containing silicon powder in slight excess of that required t-o give complete siliconising. Attack of the crucible 4by molten silicon is prevented by employing a line grain -graphite which is rendered impermeable by impregnation with furfuryl alcohol tfollowed by graphitising. Siliconising takes place when the silicon melts at 1420 C., an exothermic reaction :between the silicon and the colloidal graphite in the compacts occurs which prod-uces beta-silicon carbide which bonds the grains of alpha-silicon carbide in the compact together. Fuel bodies of matrix densities 92% of theoretical can be produced, the high densities favouring the exceptional high temperature properties of the bodies.

To equip the fuel bodies `for service in a nuclear reactor when assembled with other components into the form of yfuel elements, each :fuel body formed as aforesaid is provided with a surface layer of clean silicon carbide, that is, silicon carbide in which no :fuel spheroids are dispersed. This expedient allows a measure of grinding to dimension, should this be necessary, and also provides that no fuel body is at or near the surface of the fuel body which is swept by coolant, thereby ensuring that contamination of the coolant by fission products arising as a result of irradiation does not occur. Where the lfuel bodies are of solid elongate cylindrical form, their ends are preferably provided with a relatively thick portion of clean .silicon carbide, whereby provision, such as a recess, for location and support of each fuel body in a duel element, can be tolerated. Such surface layers may be provided by ordinary powder metallurgical methods, employing the herein-before specified materials, namely alpha silicon carbide, colloidal graphite, lubricant `and resin but without spheroids, effecting lco-extrusion employing the s-olid Ifuel rod as mandrel, and effecting selfb-onding of the silicon .carbide layer with silicon powder and heating as aforesaid. End portions of silicon carbide may be incorporated by pre-.forming them in the green state to a desired shape, [for example caps for the ends of each rod, and then bonding such caps to the rods and to the said surface layer by means of the said siliconising process.

I claim:

1. For production of a fission-product-retaining fuel body, a process comprising the steps of granulating mixed uranium dioxide and carbon, spheroidising the granules s-o formed, effecting reacti-on-sintering of the spheroids thus formed so as to convert them to spheroids of uranium carbide, coating the uranium carbide spheroids with silicon carbide, mixing the coated spheroids with silicon carbide and carbon, extruding the mixture to form compacts, and effecting self bonding of the compacts by effecting reaction between said last-added carbon and silicon to produce dense fuel bodies.

2. A process according to claim 1, including the additional steps of co-extruding each self-bonded compact with a mixture of silicon carbide and carbon employing the self-bonded compact as mandrel, an-d effecting self-bonding of the co-extr-uded silicon carbide by elfecting reaction between the `carbon mixed therewith and silicon, thereby providing a fuel-,free layer at the or each surface ori the Ifuel body which is yintende-d to be in contact with coolant when the fuel body is in operation in a nuclear reactor.

3. A process according to claim 1, including the additional step of .coating the uranium carbide spheroids with a layer of pyrolytic carbon before applying the coating of silicon carbide.

4. A process according t-o claim 1, including the additional steps of applying a coating of pyrolytic carbon to the silicon-carbide-coated uranium carbide spheroids and applying a further coating of silicon canbide `on said pyrolytic canbon coating.

References Cited bythe Examiner UNITED STATES PATENTS CARL D. QUARFORTH, Primary Examiner.

L. DEWAYNE RUTLEDGE, Examiner. 

1. FOR PRODUCTION OF A FISSION-PRODUCT-RETAINING FUEL BODY, A PROCESS COMPRISING THE STEPS OF GRANULATING MIXED URANIUM DIOXIDE AND CARBON, SPHEROIDISING THE GRANULES SO FORMED, EFFECTING REACTION-SISTERING OF THE SPHEROIDS THUS FORMED SO AS TO CONVERT THEM TO SPHEROIDS OF URANIUM CARBIDE, COATING THE URANIUM CARBIDE SPHEROIDS WITH SILICON CARBIDE, MIXING THE COATED SPHEROIDS WITH SILICON CARBIDE AND CARBON, EXTRUDING THE MIXTURE TO FORM COMPACTS, AND EFFECTING SELF HONDING OF THE COMPACTS BY EFFECTING REACTION BETWEEN SAID LAST-ADDED CARBON AND SILICON TO PRODUCE DENSE FUEL BODIES. 